Exclusive production at HERMES

1
Exclusive production at HERMES
Cynthia Hadjidakis on behalf of the HERMES
collaboration
Rencontres de Moriond, La Thuile March 28th
April 4th, 2003

• Exclusive physics and Generalized Parton
  Distributions
• Compton scattering (DVCS)
• Vector meson production
• Pion production

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Generalized Parton Distributions (GPDs)
Q2gtgt, tltlt
t
gL
p0, r0L, g ...
- Muller (1994) -
- Ji Radyushkin (1996) -
-2 x
4 GPDs defined for each quark flavour Hq
Hq conserve nucleon helicity Eq Eq
flip nucleon helicity

xx
x-x

N
N
unpolarized
polarized
3 variables x, x, t
xx
Longitudinal momentum fraction of the quark
-2x
Exchanged longitudinal momentum fraction
t
Squared momentum transfer
GPDs probability amplitude for N to emit a
parton (xx) and for N to absorb it (x-x)
3
Limiting cases and sum rules
30(DIS)
Ji sum rules
1/2 DS D Lz
( H E ) x dx Jquark
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Generalized Parton Distributions (GPDs)

• Quantum number of final state selects different
  GPDs

Vector mesons (r, w, f) unpolarised GPDs H
E Pseudoscalar mesons (p, h) polarised GPDs H
E DVCS (g) depends on both unpolarised and
polarised GPDs

• Different combination of GPDs according to the
  reaction
• flavour decomposition

Hpp0 2/3 Hu/p 1/3 Hd/p Hpp Hu/p -
Hd/p



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HERMES Spectrometer
?
?
e, e- 27.5 GeV
Tracking 57 tracking planes dP/P 2 , dq lt
0.6 mrad Particle Identification RICH, TRD,
preshower, calorimeter lepton identification
99 p identification 98 for pp gt 2 GeV
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Deep Virtual Compton Scattering e p ? e p g
BH larger than DVCS for HERMES kinematics
DVCS
Bethe-Heitler
DVCS-BH interference leads to non-zero azimuthal
asymmetry
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Azimuthal asymmetry
DVCS-BH interference gives a non zero azimuthal
asymmetry
g
- Beam helicity asymmetry - Beam charge
asymmetry
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Exclusivity for e p ? e p g
Missing Mass2 ( p g g )2
Missing Mass resolution 0.8 GeV Exclusive
region Missing Mass lt 1.7 GeV
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DVCS asymmetry measurement
e p ? e p g
e/- p ? e p g
PRL, 87 (2001), 182001
xB0.11 Q22.6 GeV2 -t0.27 GeV2
ALU -0.23?0.04(stat)?0.03(syst)
AC0.11?0.04(stat)?0.03(syst)
Expected cos F dependence
sin F dependence

• Kivel, Polyakov Vanderhaeghen (2001)

Signal of DVCS process Can be described by GPD
calculation
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HERMES upgrade the recoil detector
projection of the statistical accuracy of ALU
for 2 fb-1 (1 year of data)
Around the target reconstruct the kinematics of
the recoil particles
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Vector mesons
gL
r0L,wL,fL
p
p
e p ? e r0 (p) p p-
Missing Mass2 ( g - p - p- )2
DE (M2X-M2p)/2Mp
e p ? e w (p) p p- p0
e p ? e f (p) K K-

• Monte Carlo simulation of non-exclusive
  background
• sL/ sT separation from decomposition of decay
  angular distributions

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Cross sections for r0, f
W GeV
GPD calculation
2 gluon exchange mechanism quark exchange
mechanism

• Guichon, Guidal Vanderhaeghen (2001)

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Exclusivity for e p ? e p(n)
Missing Mass2 ( g p - p )2 Use of p- yield
to subtract the non exclusive background
p enhancement
Exclusive neutron peak position and width in
agreement with MC based on GPD model
- Mankiewicz, Piller Radyushkin (1999) -
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Longitudinal target spin asymmetry
e p ? e p n
AUL-0.18 0.05 0.02
Polarized cross section
sS ST sL SL sLT AUL sinF
xB0.15 Q22.2 GeV2 -t0.46 GeV2
sLT suppressed by 1/Q but SL gt STSsin qg
Hermes kinematics ST/S0.17
AUL ST/S AUT
2005 end of run with a tranverse polarized target
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Exclusive reaction at Hermes ongoing analysis
Run with transverse polarized target (2002-2005 )
- Goeke, Polyakov Vanderhaeghen (2001) -
- Frankfurt, Pobylitsa, Polyakov, Strikman (1999)
-
gL p ? p n
TARGET SPIN ASYMMETRY
Target spin asymmetry most promising observable
which allow to access J
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Summary and outlook

• GPDs can be probed by hard exclusive meson
  production
• Measurements at HERMES
• DVCS clear signal of the process in agreement
  with GPD
• Vector mesons longitudinal cross sections
  measurement in agreement with GPD calculation
• Pion large longitudinal target spin asymmetry
• 2005 end of transverse target runs
• ? transverse spin asymmetry accessible both for
  p and r0
• 2005 run with the recoil detector